/*
    * Copyright (C) 2009, Christian Halstrick <christian.halstrick@sap.com> and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.merge;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  
  import org.eclipse.jgit.diff.DiffAlgorithm;
  import org.eclipse.jgit.diff.Edit;
  import org.eclipse.jgit.diff.EditList;
  import org.eclipse.jgit.diff.HistogramDiff;
  import org.eclipse.jgit.diff.Sequence;
  import org.eclipse.jgit.diff.SequenceComparator;
  import org.eclipse.jgit.merge.MergeChunk.ConflictState;
  
  /**
   * Provides the merge algorithm which does a three-way merge on content provided
   * as RawText. By default {@link org.eclipse.jgit.diff.HistogramDiff} is used as
   * diff algorithm.
   */
  public final class MergeAlgorithm {
  	private final DiffAlgorithm diffAlg;
  
  	/**
  	 * Creates a new MergeAlgorithm which uses
  	 * {@link org.eclipse.jgit.diff.HistogramDiff} as diff algorithm
  	 */
  	public MergeAlgorithm() {
  		this(new HistogramDiff());
  	}
  
  	/**
  	 * Creates a new MergeAlgorithm
  	 *
  	 * @param diff
  	 *            the diff algorithm used by this merge
  	 */
  	public MergeAlgorithm(DiffAlgorithm diff) {
  		this.diffAlg = diff;
  	}
  
  	// An special edit which acts as a sentinel value by marking the end the
  	// list of edits
  	private static final Edittml#Edit">Edit END_EDIT = new Edit(Integer.MAX_VALUE,
  			Integer.MAX_VALUE);
  
  	@SuppressWarnings("ReferenceEquality")
  	private static boolean isEndEdit(Edit edit) {
  		return edit == END_EDIT;
  	}
  
  	/**
  	 * Does the three way merge between a common base and two sequences.
  	 *
  	 * @param cmp comparison method for this execution.
  	 * @param base the common base sequence
  	 * @param ours the first sequence to be merged
  	 * @param theirs the second sequence to be merged
  	 * @return the resulting content
  	 */
  	public <S extends Sequence> MergeResult<S> merge(
  			SequenceComparator<S> cmp, S base, S ours, S theirs) {
  		List<S> sequences = new ArrayList<>(3);
  		sequences.add(base);
  		sequences.add(ours);
  		sequences.add(theirs);
  		MergeResult<S> result = new MergeResult<>(sequences);
  
  		if (ours.size() == 0) {
  			if (theirs.size() != 0) {
  				EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
  				if (!theirsEdits.isEmpty()) {
  					// we deleted, they modified -> Let their complete content
  					// conflict with empty text
  					result.add(1, 0, 0, ConflictState.FIRST_CONFLICTING_RANGE);
  					result.add(2, 0, theirs.size(),
  							ConflictState.NEXT_CONFLICTING_RANGE);
  				} else
  					// we deleted, they didn't modify -> Let our deletion win
  					result.add(1, 0, 0, ConflictState.NO_CONFLICT);
  			} else
  				// we and they deleted -> return a single chunk of nothing
  				result.add(1, 0, 0, ConflictState.NO_CONFLICT);
  			return result;
  		} else if (theirs.size() == 0) {
  			EditList oursEdits = diffAlg.diff(cmp, base, ours);
  			if (!oursEdits.isEmpty()) {
  				// we modified, they deleted -> Let our complete content
  				// conflict with empty text
  				result.add(1, 0, ours.size(),
 						ConflictState.FIRST_CONFLICTING_RANGE);
 				result.add(2, 0, 0, ConflictState.NEXT_CONFLICTING_RANGE);
 			} else
 				// they deleted, we didn't modify -> Let their deletion win
 				result.add(2, 0, 0, ConflictState.NO_CONFLICT);
 			return result;
 		}
 
 		EditList oursEdits = diffAlg.diff(cmp, base, ours);
 		Iterator<Edit> baseToOurs = oursEdits.iterator();
 		EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
 		Iterator<Edit> baseToTheirs = theirsEdits.iterator();
 		int current = 0; // points to the next line (first line is 0) of base
 		                 // which was not handled yet
 		Edit oursEdit = nextEdit(baseToOurs);
 		Edit theirsEdit = nextEdit(baseToTheirs);
 
 		// iterate over all edits from base to ours and from base to theirs
 		// leave the loop when there are no edits more for ours or for theirs
 		// (or both)
 		while (!isEndEdit(theirsEdit) || !isEndEdit(oursEdit)) {
 			if (oursEdit.getEndA() < theirsEdit.getBeginA()) {
 				// something was changed in ours not overlapping with any change
 				// from theirs. First add the common part in front of the edit
 				// then the edit.
 				if (current != oursEdit.getBeginA()) {
 					result.add(0, current, oursEdit.getBeginA(),
 							ConflictState.NO_CONFLICT);
 				}
 				result.add(1, oursEdit.getBeginB(), oursEdit.getEndB(),
 						ConflictState.NO_CONFLICT);
 				current = oursEdit.getEndA();
 				oursEdit = nextEdit(baseToOurs);
 			} else if (theirsEdit.getEndA() < oursEdit.getBeginA()) {
 				// something was changed in theirs not overlapping with any
 				// from ours. First add the common part in front of the edit
 				// then the edit.
 				if (current != theirsEdit.getBeginA()) {
 					result.add(0, current, theirsEdit.getBeginA(),
 							ConflictState.NO_CONFLICT);
 				}
 				result.add(2, theirsEdit.getBeginB(), theirsEdit.getEndB(),
 						ConflictState.NO_CONFLICT);
 				current = theirsEdit.getEndA();
 				theirsEdit = nextEdit(baseToTheirs);
 			} else {
 				// here we found a real overlapping modification
 
 				// if there is a common part in front of the conflict add it
 				if (oursEdit.getBeginA() != current
 						&& theirsEdit.getBeginA() != current) {
 					result.add(0, current, Math.min(oursEdit.getBeginA(),
 							theirsEdit.getBeginA()), ConflictState.NO_CONFLICT);
 				}
 
 				// set some initial values for the ranges in A and B which we
 				// want to handle
 				int oursBeginB = oursEdit.getBeginB();
 				int theirsBeginB = theirsEdit.getBeginB();
 				// harmonize the start of the ranges in A and B
 				if (oursEdit.getBeginA() < theirsEdit.getBeginA()) {
 					theirsBeginB -= theirsEdit.getBeginA()
 							- oursEdit.getBeginA();
 				} else {
 					oursBeginB -= oursEdit.getBeginA() - theirsEdit.getBeginA();
 				}
 
 				// combine edits:
 				// Maybe an Edit on one side corresponds to multiple Edits on
 				// the other side. Then we have to combine the Edits of the
 				// other side - so in the end we can merge together two single
 				// edits.
 				//
 				// It is important to notice that this combining will extend the
 				// ranges of our conflict always downwards (towards the end of
 				// the content). The starts of the conflicting ranges in ours
 				// and theirs are not touched here.
 				//
 				// This combining is an iterative process: after we have
 				// combined some edits we have to do the check again. The
 				// combined edits could now correspond to multiple edits on the
 				// other side.
 				//
 				// Example: when this combining algorithm works on the following
 				// edits
 				// oursEdits=((0-5,0-5),(6-8,6-8),(10-11,10-11)) and
 				// theirsEdits=((0-1,0-1),(2-3,2-3),(5-7,5-7))
 				// it will merge them into
 				// oursEdits=((0-8,0-8),(10-11,10-11)) and
 				// theirsEdits=((0-7,0-7))
 				//
 				// Since the only interesting thing to us is how in ours and
 				// theirs the end of the conflicting range is changing we let
 				// oursEdit and theirsEdit point to the last conflicting edit
 				Edit nextOursEdit = nextEdit(baseToOurs);
 				Edit nextTheirsEdit = nextEdit(baseToTheirs);
 				for (;;) {
 					if (oursEdit.getEndA() >= nextTheirsEdit.getBeginA()) {
 						theirsEdit = nextTheirsEdit;
 						nextTheirsEdit = nextEdit(baseToTheirs);
 					} else if (theirsEdit.getEndA() >= nextOursEdit.getBeginA()) {
 						oursEdit = nextOursEdit;
 						nextOursEdit = nextEdit(baseToOurs);
 					} else {
 						break;
 					}
 				}
 
 				// harmonize the end of the ranges in A and B
 				int oursEndB = oursEdit.getEndB();
 				int theirsEndB = theirsEdit.getEndB();
 				if (oursEdit.getEndA() < theirsEdit.getEndA()) {
 					oursEndB += theirsEdit.getEndA() - oursEdit.getEndA();
 				} else {
 					theirsEndB += oursEdit.getEndA() - theirsEdit.getEndA();
 				}
 
 				// A conflicting region is found. Strip off common lines in
 				// in the beginning and the end of the conflicting region
 
 				// Determine the minimum length of the conflicting areas in OURS
 				// and THEIRS. Also determine how much bigger the conflicting
 				// area in THEIRS is compared to OURS. All that is needed to
 				// limit the search for common areas at the beginning or end
 				// (the common areas cannot be bigger then the smaller
 				// conflicting area. The delta is needed to know whether the
 				// complete conflicting area is common in OURS and THEIRS.
 				int minBSize = oursEndB - oursBeginB;
 				int BSizeDelta = minBSize - (theirsEndB - theirsBeginB);
 				if (BSizeDelta > 0)
 					minBSize -= BSizeDelta;
 
 				int commonPrefix = 0;
 				while (commonPrefix < minBSize
 						&& cmp.equals(ours, oursBeginB + commonPrefix, theirs,
 								theirsBeginB + commonPrefix))
 					commonPrefix++;
 				minBSize -= commonPrefix;
 				int commonSuffix = 0;
 				while (commonSuffix < minBSize
 						&& cmp.equals(ours, oursEndB - commonSuffix - 1, theirs,
 								theirsEndB - commonSuffix - 1))
 					commonSuffix++;
 				minBSize -= commonSuffix;
 
 				// Add the common lines at start of conflict
 				if (commonPrefix > 0)
 					result.add(1, oursBeginB, oursBeginB + commonPrefix,
 							ConflictState.NO_CONFLICT);
 
 				// Add the conflict (Only if there is a conflict left to report)
 				if (minBSize > 0 || BSizeDelta != 0) {
 					result.add(1, oursBeginB + commonPrefix, oursEndB
 							- commonSuffix,
 							ConflictState.FIRST_CONFLICTING_RANGE);
 					result.add(2, theirsBeginB + commonPrefix, theirsEndB
 							- commonSuffix,
 							ConflictState.NEXT_CONFLICTING_RANGE);
 				}
 
 				// Add the common lines at end of conflict
 				if (commonSuffix > 0)
 					result.add(1, oursEndB - commonSuffix, oursEndB,
 							ConflictState.NO_CONFLICT);
 
 				current = Math.max(oursEdit.getEndA(), theirsEdit.getEndA());
 				oursEdit = nextOursEdit;
 				theirsEdit = nextTheirsEdit;
 			}
 		}
 		// maybe we have a common part behind the last edit: copy it to the
 		// result
 		if (current < base.size()) {
 			result.add(0, current, base.size(), ConflictState.NO_CONFLICT);
 		}
 		return result;
 	}
 
 	/**
 	 * Helper method which returns the next Edit for an Iterator over Edits.
 	 * When there are no more edits left this method will return the constant
 	 * END_EDIT.
 	 *
 	 * @param it
 	 *            the iterator for which the next edit should be returned
 	 * @return the next edit from the iterator or END_EDIT if there no more
 	 *         edits
 	 */
 	private static Edit nextEdit(Iterator<Edit> it) {
 		return (it.hasNext() ? it.next() : END_EDIT);
 	}
 }